This invention relates to an improved cellular ceramic catalyst support.
Innumerable applications of catalytic phenomena, involving both organic and inorganic reactions, in liquid, gaseous and vapor phases, are common in modern industrial processing. Such catalytic reactions include oxidation reduction, dehydrogenation, cyclization, polymerization and dehydration reactions.
In addition to innumerable industrial processing applications, the recent emphasis on, and interest in energy conservation and pollution control has resulted in increased catalytic research activity in these areas. Such research activity into pollution control includes the control of both industrial pollution and control of emissions from internal combustion engines. Because of the rapid dependence of industrial processes upon petroleum products, and the large amount of known coal reserves in the Northern Hemisphere, research activity into efficient methods of coal gasification has increased.
Many industrial processing applications dependent upon catalytic reactions involve the use of catalysts in a static environment. In contrast, the use of catalytic materials to control exhaust emissions from internal combustion engines involve the use of such catalysts in a dynamic environment, under severe operating conditions.
Automobile manufacturers and associated industries have become increasingly concerned with the problem of controlling automobile exhaust emissions. Exhaust gases from internal combustion engines commonly contain unburned hydrocarbons, sulfur compounds, carbon monoxide, carbon dioxide and nitrogen oxides. Carbon monoxide is harmful because of its toxicity. In addition to being lacrimatory, NO.sub.2 is more toxic than equal concentration of CO. Photochemical smog, the type that occurs in California, is due to the reaction between sunlight and unsaturated hydrocarbons (olefins) with nitrogen oxide. It causes eye irritation, general discomfort, poor visibility and plant damage.
Because of an increased awareness of and concern about the deleterious effects produced by discharge of untreated and uncontrolled exhaust from internal combustion engines, automotive manufacturers and associated industries have expended considerable effort to methods which will substantially reduce or eliminate air pollution from this source. In addition, increasingly stringent controls have been imposed by government regulatory agencies which will necessitate including a reactor to more completely convert hydrocarbons, CO and nitrogen oxides to the harmless end products, water, carbon dioxide and nitrogen. Because of this increased stringency, it is estimated that in the vicinity of 330 million dollars per year is being spent on research and development in order to meet the Federal Government's air pollutant emission standards for automobiles.
Studies have shown that increasing the air to fuel ratio lowers the HC and CO emission, but causes a drastic increase in the amount of nitrogen oxide emission. At a stoichiometric (14.5/1) air fuel ratio where HC and CO emissions are low, nitrogen oxide emissions are high.
In general, two approaches to the control of exhaust emissions are used:
(a) Thermal conversion devices, and
(b) Catalytic conversion devices.
Carbon monoxide and unburned hydrocarbons are major combustibles remaining in the incompletely burned post-combustion gas mixture. These products can be mixed with secondary injection air and thermally reacted in a combustion chamber separate from the engine's spark ignition chamber to control emissions. Such thermal conversion devices can effectively reduce carbon monoxide and hydrocarbons. However, oxides of nitrogen cannot be effectively treated by thermal conversion. In addition, because of space, heat, durability and fuel economy problems, the thermal converter does not appear to be as attractive a means as the catalytic converter for controlling both those pollutants which must be oxidized (HC and CO) and those which must be reduced (NO.sub.x).
Although the ultimate nature of the catalyst used in a catalytic conversion device is not changed, it does undergo oxidation and reduction as: EQU NO+M .fwdarw. MO+1/2N.sub.2 EQU mo+co .fwdarw. m+co.sub.2
to give the effective reaction which does not proceed spontaneously without the catalyst M: EQU NO+CO .fwdarw. 1/2N.sub.2 +CO.sub.2
most catalysts are either reducers (they take oxygen from nitrogen oxides, designated as NO.sub.x, to form N.sub.2) or oxidizers (adding oxygen to unburned hydrocarbons and CO). In operation, a twin-bed system would be utilized, in which the exhaust would be routed to one converter for reduction, and to another for oxidation.
While the selection of a suitable catalyst is extremely important, the development of a catalyst support material and configuration is a prerequisite for an effective catalyst system. The catalyst carrier must possess the characteristics of high strength, high surface area, good thermal shock resistance, inertness to exhaust gas environment, inertness to the catalyst material and in addition must induce a low back pressure in the exhaust system.
Unsupported catalysts, i.e., where the catalyst is neither deposited on nor used to impregnate any carrier or support material have been suggested in the prior art. Use of such unsupported catalysts in a system having a large volume of gas flow suffers from numerous difficulties. For example the pellet beds suffer from pellet to pellet abrasion during use, causing attrition losses that result in particulate emission. In addition, the original particles of the catalyst metal or alloy tend to stick together, increasing the back pressure of the exhaust system to the extent that automobile drivability is adversely effected.
Furthermore, supported pellet beds offer low surface area for the bulk of ceramic that is necessary; the internal pores in conventional catalyst support materials do not provide useable surface at the extremely high space velocities (20,000-200,000 v/v/hour) experienced in the exhaust system.
The supported catalysts are generally supplied as spherical pellets or as monolithic structures such as honeycomb materials and foams. The honeycomb construction suffers from the disadvantage that because of straight-through flow channels, the residence time of the gases is insufficient for complete catalyzation. Foamed structures are very weak and result in excessively high back pressure in the exhaust system.